The nervous system is an extensive, intricate network of structures that activates, coordinates, and controls the functions of all other body systems. It can be grouped into two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord and is the control center of the body.
The PNS consists of the peripheral nerves, which include the cranial nerves(emerging from the base of the skull) and the spinal nerves (emerging from thespinal cord). The PNS connects the CNS to remote body parts to relay and receive messages, and its autonomic nerves regulate involuntary functions ofthe internal organs.
Despite the complex organization of the nervous system, it consists of only two principal types of cells, neurons and neuroglia. Neurons are the basic structural and functional units of the nervous system. (See Figure Neuron.) They are specialized to respond to physical and chemical stimuli, conduct electrochemical impulses, and release specific chemical regulators.
Through these activities, neurons perform such functions as the perception of sensory stimuli, learning, memory, and control of muscles and glands. Neuroglia do not carry impulses, but perform the functions of support and protection.
Many neuroglial, or glial, cells form a supporting network by twining around nerve cells orlining certain structures in the brain and spinal cord. Others bind nervous tissue to supporting structures and attach the neurons to their blood vessels.Certain small glial cells are phagocytic. In other words, they protect the CNS from disease by engulfing invading microbes and clearing away debris. Neuroglia are of clinical interest because they are a common source of tumors (gliomas) of the nervous system.
The spinal cord is a long, narrow cable of nerve tissue within the spinal canal and is part of the CNS. It descends from the brain stem to the lumbar part of the back and contains about 100 million neurons. A slightly flattened cylinder, it is about as wide as a finger for most of its length, tapering to a thread like tail. Thirty-one pairs of spinal nerves originate from the spinal cord. (See Figure Spinal nerves.) Each pair of nerves serves a specific region on the right or left side of the body.
The spinal nerves are mixed nerves that provide a two-way communication between the spinal cord and parts of the upper and lower limbs, neck, and trunk. Spin/al nerves are named according to locations of their respective vertebrae.
In Figure Spinal nerves, there are 8 pairs of cervic/al nerves,identified as C1–C8; 12 pairs of thorac/ic nerves, identified as T1–T12; 5 pairs of lumb/ar nerves, identified as L1–L5; 5 pairs of sacr/al nerves, identified asS1–S5; and 1 pair of coccyg/eal nerves, identified as Co1.Label the following nerves in Figure Spinal nerves: (1) cervical nerves; (2) thoracicnerves; (3) lumbar nerves; (4) sacral nerves; and the (5) coccygealnerve.
The space between the pia mater and the bones of the spinal cord is called the epi/dur/al space and contains blood vessels and some fat. It is the space into which anesthetics may be injected to dull pain or contrast material may be injected for certain diagnostic procedures.
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